Paths through communication systems are usually completed, or attempted to be completed, under the control of a called customer code which in one way or another is used to identify some ultimate destination toward which a call should be extended. Such calls may involve either (1) only one switching center if the destination for the call is terminated in or is directly served by the same switching center as the origination of the call or (2) any number of switching centers if the origination and destination are terminated in or are served by different switching centers.
Control of call traffic is necesary in order that attempts to complete calls do not cause either congestion of selected switching routes or excessive use of, and thus congestion of, the switching control means employed to select the switching routes.
One well-known method of controlling call traffic has been to limit in some way the amount of traffic allowed to be extended over a particular switching route, such as over a particular trunk group. This method has suffered the disadvantage that many types of calls toward many types of destinations have been subjected at times to degradation of service caused by calls toward relatively few types of destinations, such as an avalanche of calls at a particular time to or toward such destinations as telethons and the like. Limiting switching route access has generally taken the form either of a physical limitation based on attempting to route calls over a limited access route or of denying any access to a route for a period of time. When such a route (for example, a particular interswitching-center trunk group) becomes overloaded, or is otherwise denied, the control means of a switching center may route excess calls to some sort of announcement or intercept operator location from which is provided some explanation of the failure to complete the call. Such a procedure is unfair to a large majority of calls to which the blame for the congestion cannot be attributed.
Another well-known method of controlling call traffic has been to use what is known as code blocking. This method limits traffic which is attempted to be completed according to all or part of some called customer code. Here, some sort of selective control has been effected in that traffic to or toward destinations identified by particular called codes can be controlled. Such control has been rendered effective to block a certain percentage of calls either for a certain period of time or until a switching or control congestion situation clears up. This method has the disadvantage that all or a substantial number of the allowed calls may occur in a concentrated period of time, thus leaving the remaining time unused or wasted.